1. Field of the Invention
The present invention relates generally to a process for manufacturing a solar cell, and more particularly to a process for manufacturing a solar cell from a reject semiconductor wafer.
2. Description of the Prior Art
Photovoltaic cells, or solar cells as they are commonly known, have been available for many years and presently are used to annually provide 100 kilowatts (KW) of energy in the United States. However, the current market price of such cell is about $25,000.00 per kilowatt capacity, an amount which is so prohibitive that it excludes such cells from consideration as a wide spread source of residential and commercial electric power.
The cost of a starting single crystal silicon wafer is more than one half the total manufacturing cost of a finished solar cell. For example, the cost of a new 3 inch wafer is six dollars whereas the cost of a finished solar cell is about nine dollars. Hence, less expensive sources of single crystal silicon wafers must be developed. The new inexpensive silicon wafers will likely require new solar cell product fabrication techniques to minimize yield loss due to the unique features of the inexpensive starting material.
Conventional solar cells are typically fabricated on new silicon wafers which were ordered to have a specific conductivity type, dopant concentration, orientation, thickness, and diameter. New silicon wafers not satisfying the specified description are not desirable for use in fabricating conventional solar cells due to a lack of compatibility between the fabrication process and the starting wafer that would contribute to low line yield or poor product performance.
In fabricating a conventional solar cell a dopant impurity is first introduced into the front surface of the water to form a shallow P/N junction. The front and back surfaces of the wafer are then coated with a highly conductive metallic layer. Metal deposition is usually accomplished in a planetary support system in which the wafers are place above the source metal. This metal deposition technique is commonly used in the semiconductor industry because of its rapid throughput. The usual restricted use of whole wafers, thick wafers, and wafers of similar diameters facilitates the use of this method of depositing metal. The final step in the conventional solar cell fabrication procedure consists of etching a pattern in the metal layer covering the front wafer surface. This final step is accomplished through the use of the photolithographic process commonly used in the semiconductor industry of photoresist spinning, baking, image exposing, photoresist developing, metal etching and photoresist stripping. A disadvantage of this process is that it is costly and prone to low yields when used to build solar cells from nonstandard starting wafers.
In view of the high cost of conventional solar cells, the Energy Research and Development Administration (ERDA) of the United States Government has recently identified the requirement of a low cost source of silicon and a solar cell fabrication procedure compatible to such low cost silicon as an area worthy of major attention.